This invention relates to electric heating elements, for example for use in liquid heating vessels such as kettles, rice cookers, coffee makers, etc.
Various types of heating elements are known for the above applications. Until now, the most common form of heating element is the immersion element which basically comprises a metal coil which heats up when a current is passed through it.
Recently, there has been a shift to flat heating elements. In the case of electric kettles, for example, these provide the advantage that cleaning the inside of the kettle is easier, and it may be possible to boil a small quantity of water, since a smaller quantity is required to cover the heating element. One way to produce flat heating elements is simply to bond a conventional coil to the underside of a flat metal substrate. However, the use of thick film heating elements is now being explored for liquid heating applications. The invention concerns these thick film heating elements.
It is known to provide thermal sensors on this type of heating element adjacent to the thick film heating tracks. It is also possible to form the heating tracks and the thermal sensors using a single manufacturing process, when thick film thermal sensors are employed. EP-0 585 015 discloses a thick film heating element having at least one temperature sensor formed as an electrically resistive track. The temperature sensor sends signals representing overheating of the heating element, which enables the power to the element to be switched off.
A problem with temperature sensors formed adjacent the heating element tracks is that the temperature sensors do not closely follow the temperature of the material to be heated (on the opposite side of the heating element substrate), but instead are influenced by the heat produced in the heating tracks. The response of the temperature sensor to thermal changes in the material to be heated is therefore limited.